EE WAS WORKING WITH COWORKER IN THE SUB-COLLAR OF THE UCAT SHAFT. THEY WERE WORKING NEAR A MANWAY THAT HAD BEEN CUT OUT IN THE DECK TO ENABLE ACCESS TO PIPE THAT NEEDED TO BE INSTALLED. A RETRACTABLE FALL RESTRAINT WAS OVER THE HOLE AND SIGNS SHOWED OPEN HOLE. EE FELL APPROX. 12' (CONCUSSION).